Shell and tube heat exchangers have an array of tubes extending between and through two spaced apart tube sheets surrounded by a shell. The shell is provided with an inlet and an outlet so that a suitable heat exchange liquid or gas can be circulated through the shell to cool or heat a liquid flowing through each tube.
Each end of the array of tubes can be left open, or exposed, for use in some processing operations. For other operations, one or both ends can be enclosed by a liquid retaining header, which may or may not have a removable cover or access port. When only one liquid header is present, it can be either a liquid inlet or liquid outlet. When a liquid header is positioned at each end, one liquid header can constitute a liquid inlet while the other can be a liquid outlet. Such an arrangement is conventional for once-through or single pass heat exchangers. The liquid inlet and outlet headers, or portions thereof, are provided with suitable conduit means for supplying and removing liquid.
Although shell and tube heat exchangers are generally used to heat a liquid feed stream, they can be used for cooling such a stream. Shell and tube heat exchangers of the described types can be used as freeze exchangers for producing fresh water from brackish water and seawater, for concentrating fruit and vegetable juices, and in industrial crystallization processes. As the liquid flows through each tube, it can be cooled enough to crystallize a solid from the liquid. Thus, by cooling seawater, ice is obtained which when separated, washed and melted provides potable water. When a fruit or vegetable juice is similarly chilled, ice forms and is removed to provide a concentrated juice.
Heat exchangers of the described types can use any cooling fluid on the shell side to cool a liquid flowing through the tubes. The fluid can be fed through one end and removed through the other end of the heat exchanger in a substantially unidirectional flow. Some suitable cooling fluids are ammonia and Freon brand refrigerants.
To obtain optimum heat exchange it is desirable in many instances for the tubes to be arranged vertically and for the feed liquid to be supplied to the tube surfaces as a downwardly flowing or falling liquid film. Not only is the feed liquid brought more quickly close to the temperature of the heat exchange liquid in this way but less recirculation of the liquids is required, thus reducing energy consumption.
Although it has been recognized for some time that control of the thickness of the falling film is desirable to obtain maximum heat exchange efficiency, available apparatus has not provided totally acceptable results and, in addition, the equipment cost and complexity has been greater than desired. Thus, Nail U.S. Pat. No. 4,335,581 discloses a heat exchanger with stub tubes which fit loosely into the open tops or mouths of the heat exchanger tubes so that the feed liquid can only flow downwardly between the tubes. Although such apparatus may be satisfactory for small size heat exchangers, it is not a desirable arrangement for large heat exchangers.
It is customary to locate heat exchange tubes more or less side-by-side in lines parallel to the tube sheet diameter. When the upper ends of the tubes are made flush with the top surface of the upper tube sheet, liquid fed to the top of the tube sheet will generally not be distributed uniformly to each tube mouth so that a constant falling film thickness and rate are not obtained. This adversely affects the efficiency of the heat exchange.
In an effort to improve liquid distribution to the tubes, the upper ends of the tubes have been extended up to twenty-four inches above the tube sheet surface. While this has led to somewhat improved liquid distribution to the tubes, satisfactory results were not achieved in large heat exchangers because of lack of flow control to all the tube mouths when the liquid feed was fed in at one or two locations.
A need accordingly exists for an improved falling film shell and tube heat exchanger which has means to control the flow of the liquid feed so as to constantly supply liquid to each tube in a substantially uniform amount.